If they took tamoxifen for five years, then 900 deaths could be avoided meaning 2,400 women died from their cancer within 15 years.

The new findings show that if those 10,000 women took the drug for ten years then a further 300 lives could be saved. It would mean 1,200 deaths in total could be avoided meaning 2,100 women had died of their cancer at 15 years.

Writing in an accompanying comment article, Professor Trevor Powles of the Cancer Centre London, said: "If, as seems likely, the ATLAS findings will be reinforced next year, this should herald a change of practice, with the standard of care revised to 10 years rather than five years of treatment in patients for whom tamoxifen is indicated."

The trial was led by Dr Christina Davies of the Clinical Trial Service Unit and Epidemiological Studies Unit (CTSU) at the University of Oxford.

She said: "Good evidence now exists that 10 years of tamoxifen in ER-positive breast cancer produces substantial reductions in rates of recurrence and in breast cancer mortality not only during the first decade, while treatment continues, but also during the second decade, long after it has ended."

The study found a small increased risk of womb cancer by extending the use of tamoxifen.

Dr Davies added: "While our results show a small increase in life-threatening sideeffects for women who take tamoxifen for ten years rather than five, this increase is greatly outweighed by the reduction in breast cancer mortality.

"Moreover, these sideeffects cause little or no risk in pre-menopausal women with oestrogen receptor-positive disease, and if tamoxifen prevents the death of a pre-menopausal woman, then she could well gain several decades of life expectancy."

Dr Caitlin Palframan, Head of Policy at Breakthrough Breast Cancer said: “Oestrogen-positive breast cancer is the most common type of breast cancer and so improving treatment and getting the most out of the effective treatments we already have is key.